The present invention relates generally to alternators for automotive vehicles, and more particularly relates to rotors used in such alternators.
Currently, the majority of all vehicles driven today use front-end accessory drive alternators that contain Lundell style rotors, also known as xe2x80x9cclaw polexe2x80x9d rotors. The rotor provides the alternator""s magnetic field and rotates within the machine. The rotor includes a coil assembly having a field coil made up of an insulated copper wire or wires wrapped around an electrically insulating bobbin. The bobbin surrounds a steel hub, and insulates the field coil from the steel pole pieces which sandwich the field coil to form north and south poles. The magnetic field is generated when the field coil is energized and a current flows through the wires.
One problem with conventional rotors is preventing rotational movement of the field coil within the rotor assembly. The rotor is driven via a belt by the engine of the vehicle. The engine is constantly changing speeds during operation leading to accelerations and decelerations of the rotor speed. Typical vehicles experience acceleration and deceleration rates of approximately 15,000 RPM/sec with transit excursions as high as 30,000 RPM/sec. Movement of the field coil wires leads to a variety of coil failure including wire fatigue fractures, insulation abrasion, and bobbin insulator wear.
Therefore, it is critical in the rotor design to prevent the field coil from moving within the rotor assembly. Conventional solutions to this problem include locking features formed into the coil assembly and the pole pieces, as well as the use of epoxy fillers or other adhesives to attach the coil assembly to the pole pieces. For example, projections may be formed into the outside face of the bobbin that mate with indented features in the poles to help lock the bobbin and hence coil assembly in place.
Unfortunately, these locking features remove steel from the pole pieces, leading to high magnetic saturation in the poles and reducing power density. In addition, the thick locking protrusions created on the bobbin are made of plastic bobbin material that is a poor conductor of heat, preventing good heat transfer from the coil to the cooler poles and leading to an increase in field coil temperature. Likewise, the use of epoxy filler takes up space that could otherwise be filled by the field coil and prevents good heat transfer, both of which decrease the power density of the alternator. In sum, current methods of locking the field coil in position create unwanted performance tradeoffs.
Accordingly, there exists a need to provide an alternator rotor that prevents the field coil from moving within the rotor assembly while maximizing the available space for the field coil and providing increased dissipation of heat to increase the power density of the alternator.
The present invention provides an alternator rotor that prevents the field coil from moving within the rotor assembly in a manner that increases the power density of the alternator. The structure maximizes the available space for the field coil and provides increased dissipation of heat to accomplish the same. Briefly, the outer diameter of the field coil is preferably greater than or equal to the inner diameter of the first and second pole pieces for frictional engagement of the coil assembly to the first and second pole pieces. This prevents rotation of the field coil. Stated another way, the field coil defines depressions corresponding to pole fingers of the first and second pole pieces. The pole fingers are positioned within the depressions to prevent rotation of the of the coil assembly relative to the first and second pole pieces.
Preferably, the first and second pole pieces compress the field coil of the coil assembly. The field coil may be compressed radially and/or axially by the first and second pole pieces. Generally, the first and second poles include pole fingers and a pole hub, the pole fingers including a first portion extending radially from the pole hub and a second portion extending axially relative to the pole hub. The first portion and/or the second portion of the pole fingers may be positioned within the depressions, thereby preventing rotation.
The lack of any extra locking features and the compression of the field coil increases the amount of copper wire within the rotor and improves heat transfer from the field coil to the pole pieces. The improved heat transfer is a result of both increased contact pressure and increased contact area between the coil assembly and the pole pieces. All of these features improve power density of the alternator, while at the same time securely connecting the field coil to the pole pieces to prevent unwanted movement of the field coil within the rotor assembly.